Conventionally, the image forming apparatus in which an electrostatic latent image formed by exposing a charged surface of a photosensitive drum to light is developed into a toner image by using a liquid developer containing particulate toner and a carrier liquid, and the toner image developed from the electrostatic latent image is transferred onto a recording material has been known. The liquid developer is accommodated in a mixer, and is supplied from the mixer to a developing device and is used for development. Then, the liquid developer which is not used for development is collected from the developing device into the mixer, and is supplied from the mixer to the developing device again and is used again. In the developing device, by the liquid developer carried on a rotating developing roller, the electrostatic latent image formed on the photosensitive drum is developed into the toner image. Such development of the electrostatic latent image into the toner image is carried out by movement of the charged toner in a liquid layer of the liquid developer formed between the developing roller and the photosensitive drum (so-called electrophoresis).
Incidentally, a charge amount of one particle of the toner (hereinafter, this is referred to as a toner charge amount) becomes larger with an increasing surface area of the toner. Further, mobility, i.e., ease of movement of the toner in the liquid developer is proportional to the toner charge amount. Further, toners different in particle size are contained in the liquid developer. For that reason, when compared with the toner having a relatively small particle size, the toner having a relatively large particle size is easily used preferentially for development. Therefore, with progress of image formation, a ratio of the toner having the small particle size increases in the liquid developer circulating between the mixer and the developing device. However, when the ratio of the toner having the small particle size excessively increases, although a toner concentration (a ratio of the toner occupied in a total weight of the liquid developer, TD ratio) of the liquid developer is proper and the toner amount in the liquid developer is sufficient, the electrostatic latent image is liable to be developed into the toner image low in image density.
Therefore, an image forming apparatus in which toners with different particle sizes in a larger amount are used for development by enhancing development efficiency has been proposed (Patent Document 1). Here, the development efficiency is a ratio of an amount of the toner with which the electrostatic latent image is developed on the photosensitive drum to an amount of the toner in the liquid developer, and a developing property becomes higher with higher development efficiency, i.e., the electrostatic latent image is developed into the toner image with a high density. In an apparatus described in Japanese Laid-Open Patent Application 2015-55778, the toner charge amount is adjusted by charging the liquid developer carried on the developing roller by a charger, whereby the electrostatic latent image can be developed into the toner image with high development efficiency.
However, in the case where a state of the liquid developer changes with progress of image formation or an ambient environment such as a temperature or a humidity change, a charge state of the toner is influenced, so that the toner charge amount can change. In that case, in the apparatus described in Japanese Laid-Open Patent Application 2015-55778, it becomes difficult to properly adjust the toner charge amount. In that case, the high development efficiency cannot be maintained and a ratio of the toner having the small particle size in the liquid developer increases, and sooner or later, the developing property lowers and the electrostatic latent image is developed into the toner image with a low image density. Further, the apparatus has to be upsized in order to ensure a space in which the charger is provided, and this is contrary to a recent demand for downsizing and it is difficult to employ the apparatus also from the viewpoint of a cost.
An object of the present invention is to provide an image forming apparatus capable of developing an electrostatic latent image into a toner image while maintaining high development efficiency in a simple constitution and thus capable of suppressing the development of the electrostatic latent image into the toner image low in image density.